feat(ui): support Progress custom format

[wyf027]

Summary

• Add a format prop to Progress so consumers can customize visible progress text, matching mature component APIs.
• Use string/number formatted output as the default aria-valuetext, while keeping explicit aria-valuetext as the override path for richer announcements.
• Cover custom formatting in tests and update the VitePress example to use Tailwind layout classes.

Validation

• GitHub Actions CI run 28908836271 passed for commit 6b4c78d5a4875490678d2495bb37156573cb21c4.
• CI quality job passed: Install, Lint UI, Typecheck UI, Test UI, Build UI, Build docs.
• Local npm run lint:ui, npm run typecheck:ui, npm run test:ui, and npm run build could not be executed because the local shell runner failed to create a process in this Codex environment.